1. Field of the Invention
The present invention relates to nuclear fuel assemblies and, more particularly, to fuel assemblies suitable for boiling water reactors.
2. Description of the Prior Art
A known fuel assembly used for a boiling water reactor is provided with upper and lower tie plates, a plurality of fuel rods whose upper and lower ends are held by the upper and lower tie plates, respectively, and fuel spacers for maintaining the spacing between adjacent fuel rods, as described in Japanese Laid-Open Patent Application JP-A-60-120282. The bundle of fuel rods is inserted into a channel box, which is attached to the upper tie plate. The lower tie plate is slidably inserted into the lower end portion of the channel box. Finger springs are disposed between the channel box and the lower tie plate. The finger springs suppress the leakage of the coolant in the channel box to the outside via the gap between the channel box and the lower tie plate.
The finger spring of this conventional fuel assembly cannot completely prevent the coolant in the fuel assembly from leaking to the outside from the gap between the channel box and the lower tie plate. The conventional fuel assembly is therefore defective in that the amount of coolant leaking from the gap cannot be controlled.
There is a risk of breakage of the finger springs. Additionally, the present inventors have found that the pressure difference between the inside and outside of the channel box causes creep deformation which increases the leakage rate over the exposure time of the fuel rods.
The finger spring applies a force to the channel box that is directed outwardly. This force promotes creep deformation at the lower end portion of the channel box and expands the lower portion of the channel box outwardly.
Japanese Laid-Open Patent Application JP-A-61-170692 discloses a structure that does not use the finger spring. As is shown in FIG. 1 of this prior art reference, steps having an inward inclination are disposed around the outer peripheral portion of the lower tie plate and the channel box is placed on one step of this structure. The channel box is thus kept in close contact with the lower tie plate by its own weight and this restricts the leakage of cooling water. Since the channel box is placed on the steps having an inward inclination, inward components of force are caused to act on the lower end portion of the channel box and outward deformation of the outside of the lower end portion is reduced.
However, force still acts on the lower end portion of the channel box from inside to outside due to the pressure difference between the inside and outside of the channel box and this is likely to promote creep deformation. If the lower end portion of the channel box expands, the quantity of leaking cooling water increases.